Photoelectric conversion elements are used in various photosensors, copying machines, solar cells, and the like. These photoelectric conversion elements have adopted various systems to be put into use, such as elements utilizing metals, elements utilizing semiconductors, elements utilizing organic pigments or dyes, or combinations of these elements. Among them, solar cells that make use of non-exhaustive solar energy do not necessitate fuels, and full-fledged practicalization of solar cells as an inexhaustible clean energy is being highly expected. Under such circumstances, research and development of silicon-based solar cells have long been in progress. Many countries also support policy-wise considerations, and thus dissemination of silicon-based solar cells is still in progress. However, silicon is an inorganic material, and has limitations per se in terms of throughput and molecular modification.
As a next-generation technology to solve such problems as described above, research is being vigorously carried out on dye-sensitized solar cells. Particularly, Graetzel et al. at l'Ecole Polytechnique de l'Universite de Lausanne in Switzerland have developed a dye-sensitized solar cell in which a dye formed from a ruthenium complex is fixed at the surface of a porous titanium oxide thin film, and have realized a conversion efficiency that is comparable to that of amorphous silicon. Thus, they instantly attracted the attention of researchers all over the world.
U.S. Pat. Nos. 5,463,057, 5,525,440 and JP-A-7-249790 (“JP-A” means unexamined published Japanese patent application) describe dye-sensitized photoelectric conversion elements making use of semiconductor fine particles sensitized by a dye, to which the foregoing technology has been applied. These dye-sensitized photoelectric conversion elements are produced by applying a high-viscosity dispersion liquid containing semiconductor fine particles on an electrode support, volatilizing the solvent from the applied dispersion liquid at a relatively high temperature (e.g., 400° C. to 500° C.), and adsorbing a dye thereto. However, the time or energy consumed in this solvent volatilization process poses an obstruction to cost reduction. Furthermore, since the type of the electrode support that supports the semiconductor fine particle layer is limited, it is difficult to form an electrode layer on a plastic substrate or the like.
In regard to this problem, JP-A-2002-280587 describes a method of adsorbing a ruthenium complex dye to semiconductor fine particles, by applying on a support a dispersion liquid in which the content of additives excluding semiconductor fine particles and dispersion solvent is 1% by mass or less of the dispersion liquid, and heating the dispersion liquid coating at 250° C. or below. However, the ruthenium complex dyes used in the sensitized dyes are very expensive. Furthermore, there are concerns about the supply of ruthenium, and it still cannot be said that this technology is satisfactory as a next-generation technology supporting clean energy to cope with the above-described problems in a full-fledged manner. Rather, the research and development intended for practicalization has been just begun to some extent.
For such reasons, development of a photoelectric conversion element which is sensitized by an organic material that is inexpensive and is less restricted in resources, and which has sufficient conversion efficiency, is desired. Reports are beginning to emerge on the use of organic dyes as sensitizers of a photoelectric conversion element (see JP-A-2008-135197). However, this is a method for forming a porous semiconductor fine particle layer at a high temperature of 500° C.